Amplifiers are known to be either linear or nonlinear. Linear amplifiers are often used to amplify amplitude varying signals, such as amplitude modulated (AM) signals. Nonlinear amplifiers are often used to amplify constant amplitude signals, such as frequency modulated (FM) signals.
One known type of linear amplifier is the Doherty-type amplifier. The Doherty-type amplifier was originally developed to improve the efficiency of high power vacuum tube transmitters broadcasting in the AM frequency band. The Doherty concept was initially extended to radio frequencies, and with the advent of gallium arsenide transistors, has now been extended to microwave frequencies. The extension of Doherty-type amplifiers to radio frequencies is generally described in a publication entitled "Efficiency of Doherty RF Power-Amplifier Systems," authored by F. H. Raab, and published in the Institute of Electrical and Electronics Engineers (IEEE) Transactions on Broadcasting, Volume BC-33, Number 3, September 1987, at pages 77-83. One example of the extension of Doherty-type amplifiers to microwave frequencies is provided in U.S. Pat. No. 5,420,541 issued to Upton et al. Both of these references are incorporated herein by reference.
A block diagram of a typical RF Doherty-type amplifier 100 is shown in FIG. 1. The Doherty-type amplifier includes a carrier amplifier stage 101, a peaking amplifier stage 103, a signal splitter 105, a signal combiner 107, and a phase-matching transmission line 109. The carrier amplifier stage 101 typically includes a class AB carrier amplifier 111 and associated input and output matching circuits 113, 115. The peaking amplifier stage 103 typically includes a class C peaking amplifier 117 and associated input and output matching circuits 119, 121. The signal splitter 105 typically comprises a three decibel (dB) splitter that includes two 70.7 ohm, one-quarter wavelength transmission lines 123, 124 and a 100 ohm isolation resistor 126. The signal combiner 107 typically comprises a one-quarter wavelength transmission line transformer having a nominal characteristic impedance of 70.7 ohms. The phase-matching transmission line 109 comprises a one-quarter wavelength transmission line having a nominal characteristic impedance of 50 ohms and is used to offset the one-quarter wavelength phase shift introduced by the signal combiner 107.
When an amplitude-varying input signal 128, such as the exemplary AM signal depicted in FIG. 2, is applied to the amplifier input, the signal splitter 105 divides the input signal power equally between the carrier amplifier stage 101 and the peaking amplifier stage 103. The signal splitter 105 also isolates the amplifier stages 101, 103 from one another via its isolation resistor 126. When the amplitude of the input signal 128 is at or below the envelope average 201 (P.sub.AVG in FIG. 2), the peaking amplifier stage 103 remains in an "off state" resulting from its class C bias operation. In the off state, the peaking amplifier stage 103 presents a high impedance to the junction 129 of the 50 ohm load 130 (e.g., an antenna) and the signal combiner 107, thereby presenting minimal loading to the carrier amplifier stage 101. In addition, when the peaking amplifier stage 103 is the "off state," the 50 ohm load 130 presents an effective load of 100 ohms to the carrier amplifier stage 101 due to the impedance transformation properties of the signal combiner 107. The output matching network 115 of the carrier amplifier stage 101 is typically optimized to operate the class AB carrier amplifier 111 near saturation when the input signal level is at the envelope average 201 and the output matching network 115 is loaded with 100 ohms. Since high efficiency occurs near saturation, the carrier amplifier stage 101 alone operates very efficiently when the input signal amplitude 128 is at the envelope average 201.
As the input signal level rises above the envelope average 201 toward a desired peak envelope power (PEP) level 203, the peaking amplifier 117 is driven out of cutoff and the peaking amplifier stage 103 begins sourcing current into the combining junction 129. As the operation of the peaking amplifier 117 moves from cutoff to saturation, the effective impedance perceived by the carrier amplifier stage 101 decreases from 100 ohms (when the peaking amplifier 117 is cutoff) to a value of 50 ohms (at PEP drive when the peaking amplifier 117 is operating in saturation). This twofold reduction of load impedance allows the carrier amplifier stage 101 to source twice its former power level, while remaining near saturation and operating efficiently. Thus, at PEP drive, the carrier amplifier stage 101 provides one-half of the amplifier's desired PEP output power. The peaking amplifier stage 103 is designed to produce the remaining one-half of the amplifier's PEP output power requirement. In practical application, the carrier amplifier 111 and the peaking amplifier 117 are often the same active device (e.g., radio frequency (RF) power transistor). However, because of their different classes of operation, they are operated at different biases. The operation at different biases often results in the two amplifiers 111, 117 providing unequal gains, with the gain of the carrier amplifier 111 being greater than the gain of the peaking amplifier 117. Further, practical designs of the output matching network 115 that optimize efficiency at the amplifier's envelope average output power frequently result in higher gain of the carrier amplifier stage 101 as compared to the gain of the peaking amplifier stage 103. Thus, the carrier amplifier stage 101 most often produces a larger proportion of the output PEP power than does the peaking amplifier stage 103.
At PEP drive, the two 100 ohm load impedances (one presented to the output of the peaking amplifier stage 103 and the other presented to the output of the combiner 107 through transformation of the 50 ohm impedance presented to the carrier amplifier stage 101) are in parallel and, thus, match the 50 ohm amplifier load impedance 130. As is well known, optimal power transfer occurs when an amplifier's output impedance is matched to (i.e., a complex conjugate of) the amplifier's load impedance. Further, with the Doherty configuration, both amplifier stages 101, 103 maintain operation near saturation with corresponding high efficiency when the input signal 128 is at the PEP drive level 203. Thus, as described above, the Doherty-type amplifier 100 employs a level sensitive load impedance modulation technique to maintain saturated amplifier stage operation over a six dB peak-to-average ratio of output powers.
Although the Doherty-type amplifier 100 has many advantages, it also has some disadvantages, especially when such an amplifier 100 is evaluated for use in portable wireless applications, such as portable radios and radiotelephones. The marketplace is forcing portable wireless devices to be physically smaller and more efficient to increase talk-time available from low power battery sources. However, the typical Doherty-type amplifier 100 does not lend itself easily to small size implementation. In particular, the transmission lines 123, 124 required for the signal splitter 105 and the phase-matching transmission line 109 can be quite lengthy (e.g., 2.2 inches on a printed circuit board having a dielectric constant of 4.0 when the operating frequency is 800 Megahertz (MHz)). In addition, the use of a 3 dB signal splitter 105 wastes one-half of the input signal power when the input signal amplitude is at or below the envelope average 201 because the peaking amplifier 117 is cutoff and equal input power is provided to each amplifier stage 101, 103 at all times. This waste in power results in lower effective amplifier gain and, as a result, lower efficiency for a multi-stage amplifier lineup (not shown) that includes the Doherty-type amplifier 100 as a final amplifier stage of such lineup, even though the Doherty-type design maintains operation of the amplifiers 111, 117 themselves in high efficiency modes.
Therefore, a need exists for an amplifier that provides the advantages of the Doherty-type amplifier 100, while improving amplifier gain--and as a result amplifier lineup efficiency--and being small in physical size to facilitate use in portable wireless devices.